Telecommunication networks where signals are transmitted along optical channels are well known in the art. Unfortunately, these networks suffer occasionally from faults occurring in one of these channels, e.g. due to failing components. Therefore, a protection mechanism is usually incorporated in such systems, allowing the diversion of transmitted communication to a non-failing path, the protection path. Traditionally, monitoring the performance in these telecommunication systems was done while incorporating various alarm conditions. Such alarm conditions alerted a human operator when certain events e.g. a loss of signal or error rates that had exceeded pre-defined thresholds, were detected. In response to such an alarm, the operator would manually switch to a redundant path in the network, allowing the communication to continue.
At a later stage, conventional fiber optic networks have implemented 1:1 redundancy for optical channels extending between two locations in the network, with a certain amount of automatic switching. In these systems, when a loss of signal (to be referred to hereinafter as “LOS”) or alarm indication signal (“AIS”) conditions are noted, the transmission is diverted to an available redundant path extending between the two locations. Such diversion allows the transmission of signals between these first and second locations to continue.
U.S. Pat. No. 4,646,286 discloses a system wherein a protection switch is effected by detecting a channel failure at receiving end. Thereafter, a protection request is transmitted on the return channel to the transmission end. This request is then used in a controller for the channel to activate a switch to the corresponding protection channel.
Another solution was described in U.S. Pat. No. 5,479,608 that discloses a telecommunication system having 1:N group protection. By this type of solution one redundant channel is allocated to protect a number of operative channels. According to this solution, in response to the detection of an error condition, a request is transmitted to the other side of the system to activate the protection channel.
Our co-pending application U.S. Ser. No. 09/500,823 describes a method for diverting traffic if the energy received over the reception optical link at the second location does not exceed a pre-defined threshold.
In a paper submitted by NEC USA, Lucent Technologies, Nortel Technologies and Telcordia technologies to the International Telecommunication Union (“ITU-T”) on February 2000, a method is described for carrying out an optical channel ring protection. Their method requires two fibers for each span of the ring. Each of the fibers carries both working channels (working wavelengths) and protection channels (standby wavelengths). On each fiber, half the channels are defined as working channels and the other half are defined as protection channels, where both the working channels and the protection channels are transmitted at the same direction.
Similarly, Recommendation G.841 of ITU-T dated October 1998 describes under paragraph 7.2 entitled “MS shared protection rings” the use of two fibers for each span of the ring. Again, each of the fibers carries both working channels (working wavelengths) and protection channels (standby wavelengths). The normal traffic carried on working channels in one fiber are protected by the protection channels travelling in the opposite direction along the other fiber.
In a paper submitted by Alcatel to the International Telecommunication Union (“ITU-T”) on Sep. 25, 2000, a method for carrying out an optical channel shared protection ring. By the method described two optic fibers are used, each divided into 2 OMS Optical Multiplex Sections (“OMS”). The traffic transmitted in one of the fibers is transmitted clockwise while in the other fiber counter-clockwise. On each fiber half of the available optical channels are used for the working traffic and the other half is used for the protection. This architecture allows that when a fiber breaks, its working channels can be restored on the protection OMS of the other fiber which is counter-propagating.
However, the protection methods described in the art provide a full trail or ring solution, depending on the network architecture chosen. These methods fail to provide a solution to a case where traffic can be diverted to the protection path only for a segment of the network and then reverted to the main path. In particularly, when a ring configuration is used, and the main path is out of order at a certain location, the methods provided by the prior art call for re-routing the traffic to the protection path.
The disclosure of all references mentioned above and through the present specification is hereby incorporated herein by reference.